Conventionally, polyether silicones are obtained by addition reactions of hydrogen siloxanes and polyoxyalkylenes (polyethers) containing terminal double bonds in the presence of noble metal catalysts such as chloroplatinic acid.
However, when the polyether silicones thus obtained are used in an emulsion system, they tend to generate an odor and are therefore difficult to incorporate in cosmetics. These disadvantages were thought to be due to the fact that the polyether silicones suffered oxidative degradation to produce odorous aldehydes with the passage of time.
The present inventors studied the mechanism of this acquisition of odor in more detail and found that the odor was due to an internal rearrangement side reaction of the polyoxyalkylene which occurred in the presence of the platinum catalyst employed to produce the silicones so that it does not react with the hydrogen siloxane and remains in the polyether silicone produced by the main reaction, and is also due to unreacted polyoxyalkylene remaining in said polyether silicone. These impurities decompose with time to produce odorous ketones and aldehydes.
When, for example, allyl ether polyether is used as the polyoxyalkylene and is reacted with a hydrogen siloxane in the presence of a platinum catalyst, the allyl group of a portion thereof undergoes an internal rearrangement as a side reaction to produce propenyl ether polyether. This propenyl ether polyether does not react with the hydrogen siloxane and therefore remains as an impurity in the polyether silicone produced by the main reaction. Unreacted allyl polyether also remains as an impurity in the polyether silicone. The unreacted allyl polyether is also gradually isomerized by residual platinum catalyst remaining in the polyether silicone into propenyl ether polyether.
Apparently when water or atmospheric moisture contacts a polyether silicone containing these impurities, the propenyl ether is cleaved, and propionaldehyde, which has an unpleasant odor, is produced. This reaction is accelerated in the presence of acid and the lower the pH the greater the reaction rate.
The present inventors, knowing the above odor-producing mechanism, determined that by treating polyether silicones containing said impurities with water or an aqueous solution of pH no greater than 7 or an acidic material until all of the odorous substances are produced and then removing these odorous substances, the thus-purified polyether silicones no longer acquired any unpleasant odor with the passage of time or when contacted with water.